For the transmission of digital data, multi-channel data transmission based on Orthogonal Frequency Division Multiplex (OFDM), also referred to as Discrete Multitone (DMT) modulation, is a known flexible modulation scheme. OFDM spreads the data to be transmitted over a large number of sub-carriers or sub-channels which are contained in a transmission band and separated from each other by well-defined frequency spacing or carrier separation. The latter can ensureorthogonality of the sub-carriers and prevent crosstalk or inter-carrier interference between sub-carriers. As such, the demodulator for one sub-carrier is not affected by the modulation of the other sub-carriers even though there is no explicit filtering of the sub-carriers and their spectra overlap. The individual OFDM modulation symbols on each of the carriers represent a number of bits that depends on the choice of the QAM alphabet, (i.e., the arrangement of data or constellation points in the quadrature amplitude plain). For instance, a 2 bit/symbol for Quadrature Phase Shift Keying (QPSK), or 4 bit/symbol for 16-QAM (Quadrature Amplitude Modulation) is commonly used. The complex processes of modulating and demodulating thousands of carriers simultaneously are comparable to Discrete Fourier Transform operations, for which efficient Fast Fourier Transform (FFT) algorithms exist.
A suitable modem architecture comprises an encoder to multiplex, synchronize and encode the data to be transferred, as well as a modulator to form a discrete multitone signal. The encoder translates incoming bit streams into in-phase and quadrature components for each of a multiplicity of sub-channels. In particular, the encoder outputs a number of sub-symbol sequences that are equal to the number of sub-channels available to the system. A line monitor at a receiver end repeatedly checks the line quality of the sub-channels by determining the noise-level, gain and phase-shift on each of the sub-channels during use. The bit error rate and the signal-to-noise ratio are then used to dynamically determine the bit transmission rate that the sub-channels can support.
OFDM is, for example, suitable for Power Line Communication (PLC). Power line channels at a high or medium voltage (e.g., above 1 kV) are affected by interferers, because the cable types that are used for the transmission of electric power are unshielded and are therefore vulnerable to electromagnetic ingress. A known noise scenario on power line channels resulting therefrom includes so-called narrowband interferers (i.e., signals with a small bandwidth). In addition, the heterogeneous structure of the power line network with numerous branches and impedance mismatching causes numerous reflections (echoes) and multi-path propagation between the transmitter and receiver. In the presence of multi-path propagation, the complex transfer function h(i) of a power line link between the transmitter and receiver is a sum over a number of paths. In addition, power cables exhibit signal attenuation increasing with length and frequency selective fading.
Intersymbol Interference (ISI) is caused by the interaction of one symbol or waveform with other symbols in time. Multi-path induced ISI can be reduced by the provision of a guard interval. Each modulation symbol is transmitted for a total symbol period TOFDM which is longer than an active symbol period TORTH by a period called the guard interval TGUARD. This implies that the receiver will not experience any inter-symbol interference, provided that any echoes present in the signal have a delay which does not exceed the guard interval. Naturally, the addition of the guard interval reduces the data capacity by an amount dependent on its length, which prohibits its application to a single-carrier system.
A method of shortening a channel Impulse Response IR is disclosed in DE 44 02 512 C1. To shorten the overall channel IR of a communication channel comprising a send filter, a physical power line link between transmitter and receiver, and an input or receive filter, a receive filter transfer function is synthesized after channel estimation during a handshaking procedure.